The present invention relates to printing apparatus, and also to a substrate feeding system particularly useful in such apparatus. The invention is especially useful in high-speed ink-jet printing apparatus for printing large, multi-colored signs, and is therefore described below with respect to such an application.
One of the particularly difficult problems in high speed printing apparatus is the need to maintain the proper tension on the substrate as it is fed through the apparatus. The amount of tension to be applied to the substrate depends to a large extent on the nature of the substrate itself. If the tension is too low, the substrate may not feed properly through the apparatus, and/or the printing system may not properly print on the substrate in the printing station. On the other hand, if the tension is too high, the substrate may rupture, resulting in considerable wastage of materials and production time on the apparatus.
Another problem encountered particularly by ink-jet printing apparatus is the difficulty to maintain the ink-jet modules clean so as to minimize the possibility of blockage. Thus, when a jet-ink printer is turned-off, e.g., between printing runs, the ink within the ink supply system may dry, producing solid particles which can clog the fine orifices of the nozzles.
A further problem involved in high-speed printing by ink-jets is the need to print with relatively high resolution, and without smear, at relatively high printing rates. In conventional jet printers, high resolution may be obtained by decreasing the printing speed, but this substantially decreases the output of the apparatus. On the other hand, if the printing speed is increased In order to increase the output printing rate, this not only reduces the resolution, but also tends to produce smear in the printed marks because of the high differential velocity between the print heads and the substrate.
One object of the present invention is to provide a substrate feeding system particularly for printing apparatus, which system addresses the above-described substrate-tension problem in a novel and efficient manner. Another object of the present invention is to provide printing apparatus, particularly of the ink-jet type, which enables the nozzles to be cleaned when desired in an efficient manner in order to minimize the possibility of nozzle clogging by solid ink particles within the ink supply. A further object is to provide an inkjet printer which permits printing at high speed with high resolution and with no smear.
According to one aspect of the present invention, there is provided printing apparatus for printing on a substrate, comprising: a substrate feeding system for feeding the substrate through a printing station in the apparatus; and a printing system at the printing station for printing on the substrate; the substrate feeding system including; a drive roll upstream of the printing station; a drive motor for driving the drive roll; a tension roll downstream of the printing station; a tension motor for driving the tension roll; a coupling assembly coupling the tension motor to the tension roll and effective to apply a pivotal force to the tension motor in one direction about the axis of the tension roll according to the actual tension on the substrate; and a biasing-force device effective to apply a pivotal biassing force to the tension motor in the opposite direction about the axis of the tension roll according to the desired tension on the substrate.
According to further features in the preferred embodiment of the invention described below, the biassing-force device includes a piston displaceable within a cylinder, one of which is fixed and the other of which is coupled to the tension motor to apply the pivotal force in the opposite direction from that applied to the tension roll by the actual tension on the substrate. In the described preferred embodiment, the cylinder and piston define between them a fluid chamber having presettable means for presetting the fluid pressure therein, and thereby the desired tension on the substrate.
According to further features in the described preferred embodiment, the tension motor is coupled to the tension roll by a worm driven by the tension motor, meshing with a gear fixed to the tension roll. The gear is enclosed within a gear housing, and the tension motor is enclosed within a motor housing fixed to the gear housing, the motor housing being pivotal about the axis of the tension roll.
According to still further features in the described preferred embodiment, the drive motor drives the drive roll in steps, and the tension motor drives the tension roll in steps in synsyronism with the drive motor. The substrate feed system further includes a supply roll upstream of the drive roll, the supply roll including a restraining device for restraining tie rotation of the supply roll, and an encoder device for providing an indication of the amount of substrate paid out to the drive roll The drive motor drives the drive roll in the longitudinal direction of the substrates and tic printing system includes a printing head assembly which is driven in the transverse direction of the substrate during the intervals between the stepped movements of the substrate in the longitudinal direction. The printing head assembly includes at least one print head having a linear array ink nozzles arranged in the longitudinal direction of the substrate.
According to another aspect of the present invention, there is provided printing apparatus for printing on a substrate, comprising a substrate feeding system for feeding the substrate through a printing station in the apparatus, and a printing system at the printing station for printing on the substrate, which printing system comprises at least one print head having a linear array of ink-jet nozzles arranged in the longitudinal direction of the substrate. The printing system further includes: a common reservoir for all the nozzles in the print head; an ink supply; a solvent supply; a fluid pressure supply; a purging outlet; an ink pump for pumping ink from the ink supply to the common reservoir; a solvent pump for pumping solvent from the solvent supply to the common reservoir; and control devices for controlling the printing system to selectively connect: (a) the ink pump to the common reservoir to feed ink to the common reservoir; (b) the solvent pump to the common reservoir to feed solvent to the common reservoir; (c) the fluid pressure source and the ink supply to the common reservoir, when containing ink, to return the ink to the ink supply; and (d) the fluid pressure source and the purging outlet to the common reservoir, when containing solvent, to discharge the solvent through the purging outlet.
According to a further aspect of the present invention, there is provided printing apparatus for printing on a substrate, comprising: a substrate feeding system for feeding the substrate through a printing station in the longitudinal direction of the substrate; and a printing system at the printing station movable over the substrate in the transverse direction with respect to the substrate; the printing system comprising a printing head assembly which includes at least three pairs of print heads each provided with a linear array of nozzles for printing with the same color ink; the nozzles of each print head of each pair being staggered with respect to the nozzles of the other print head of the pair; the nozzles of each pair of print heads being staggered with respect to the nozzles of the other pairs of print heads.
According to a still further aspect of the present invention, there is provided a substrate feeding system for feeding a substrate, comprising: a drive roll and a drive motor for driving the drive roll; a tension roll and a tension motor for driving the tension roll; a coupling assembly coupling the tension motor to the tension roll and effective to apply a pivotal force to the tension motor in one direction about the axis of the tension roll according to the actual tension on the substrate; and a biassing-force device effective to apply a pivotal force to the tension motor in the opposite direction about the axis of the tension roll according to the desired tension on the substrate.
As will be described more particularly below, the foregoing features of the present invention address the tension-control problem, the printing-speed problem, and the nozzle-cleaning problem, in an efficient manner which is particularly useful in high-speed printers for printing large multi-colored signs.
Further features and advantages of the invention will be apparent from the description below.